1. Field of Invention
This invention relates to a method of retarding the hydration of cement. Specifically, it relates to a method of heat treating cement to retard its rate of hydration after mixing with coarse and fine aggregate and water to form concrete.
2. Prior Art
The chemical reactions taking place during the hydration of cement, such as portland cement, are exothermic, i.e. they evolve heat. These chemical reactions include the hydration of calcium silicates and calcium aluminates. When large masses of concrete are in the process of hardening, or curing, or during hot weather, the temperature of the concrete may rise appreciably, further accelerating the rate of chemical reaction, or curing, in the concrete and accelerating the rate of heat evolution. This heat evolution may raise the temperature of the concrete to the boiling point of water, vaporizing a portion of the water in the mixture, disrupting the concrete, and producing an unacceptable concrete.
It is important, therefore, to provide methods of controlling the rate of hydration, and thus the rate of heat evolution, during the curing of cement. Conventionally, this is accomplished by the addition of chemical retarders to the cement, aggregate and water mixture.
Chemical retarders currently in use include calcium, sodium, or ammonium lignosulfonates; organic acids including adipic, citric, and tartaric acids or their sodium salts; carbohydrates and their derivatives such as sugars, starch, and sodium gluconate; and inorganic phosphate and borate salts. Such chemical retarders are typically added in small proportions, e.g. 0.2-0.5 percent by weight based on cement. Small variations in the dosage of retarders from the norm can produce disastrous effects, such as cement that will not hydrate and cure. Chemical retarders, even when used exactly in the prescribed dosages, can interact chemically with other additives used for improving the pumpability of the concrete, producing unexpected side effects, such as an acceleration of the hardening process rather than a slowing down of this process, or conversely, concrete mixtures that will not harden at all.
Some retarders are toxic and potentially carcinogenic and therefore pose a health and safety risk during use. Retarder formulations containing formaldehyde pose a particular health risk.
There is a need, therefore, for an improved method of retarding the rate of hydration of cement. This is accomplished in the method of this invention by subjecting the cement briefly to a high temperature treatment.
High-temperature processes in the manufacture of cements, such as portland cement are known to the art. U.S. Pat. Nos. 4,213,791, 4,249,952, 4,289,537, and 4,388,117 disclose processes in which the ingredient mixtures for making the cerment, generally limestone, sources of silica, and-optionally, sources of alumina and ferric oxide, are heated above the fusion temperature (&gt;1500.degree. C.) of such materials in electric arc furnaces, whereby pools of molten material are produced having the desired chemical compositions. Molten material from these pools is intermittently or continuously tapped and allowed to cool and solidify. Ingredients for making the cement are added to the furnaces continuously or intermittently. U.S. Pat. Nos. 4,333,766 and 4,477,283 disclose processes in which plasma arc torches are used to raise the temperature of the cement ingredients above their melting points. There is no indication in any of these prior-art high-temperature treatment processes that cement having a slow rate of hydration during curing is being produced.